1. Field of the Invention
This invention relates to an apparatus for mixing a powder with a liquid, and in particular, to a mixing jar for quickly mixing components of a fast-setting, flowable molding compound.
2. Description of the Related Art
People create duplicates of objects for many reasons including art, prosthetics, and mass production. Often the first step is the creation of a "negative impression" of the object by pressing the object into a deformable molding compound that reshapes itself against the surface of the object and holds the new shape after the object is removed. Afterwards reproductions of the object may be produced by pouring a substance into the negative impression and allowing the substance to harden. The substance's shape is then that of the original object.
Alginates are used as a fast-setting flowable molding compound in dentistry. Irreversible hydrocolloids, more commonly called alginates, were developed during World War II. Alginates are salts of alginic acid, a hydrophylic colloidal polysaccharide. Alginic acid, in the form of mixed salts with sodium, calcium, magnesium, and other bases, makes up a large portion of the cell walls of marine kelp, and is a by-product in the preparation of iodine from kelp. Alginates are changed from soluble form to insoluble gel form by an irreversible chemical reaction.
The essential constituents of alginate impression powders are the soluble alginate, a reactor such as calcium sulfate, and a retarder such as trisodium phosphate. When this powder is mixed with water, all three ingredients proceed to dissolve. The retarder, however, ties up the reactor for a short time so that the reactor is not available to convert the soluble alginate to an insoluble gel. After the retarder is spent, gelation begins throughout the powder-water mixture. Among the patents disclosing alginate impression compounds are U.S. Pat. No. 3,010,834 (Crowell) and U.S. Pat. No. 2,249,694 (Wilding). The setting times of commercially-available alginate impression materials range from about 1 minute to about 4 minutes.
The powder-water mixture preferably achieves a uniform pasty gel-like flowable consistency. It cannot be too soupy, nor too dry, and powdery "pockets" in the mix are unsatisfactory. The mixture must be provided in the correct proportions and then mixed thoroughly to ensure that the water is evenly distributed. During mixing, clumps containing dry or partially dry powder tend to form in the mix. These clumps resist the introduction of water and must be broken to allow the water to reach the powder they contain. One way of accomplishing the mixing is in a bowl with a spatula. This method tends to be messy and to require a certain level of skill. The mixing must be gentle enough to avoid scattering powder and yet vigorous enough to ensure thorough distribution of the water to all the powder. A somewhat costly alternative is to buy a machine to do the mixing.
The mixing problems are exacerbated as the required amount of molding compound increases. The greater the amounts of powder and water, the greater the tendency of clumps to form. Increased agitation time is not a feasible alternative due to the fast setting times of the alginate compounds. Mixing small batches and adding the molding compound to the mold one batch at a time leads to seam lines in the mold and may even lead to separation of the mold along the interfaces between batches. An inexpensive apparatus for quickly mixing components of a fast-setting, flowable molding compound is therefore desirable.
In addition, it is sometimes desired to create a molded object by filling the mold formed by the fast-setting molding compound with a second molding compound. The second molding compound may form a more permanent material that sets into a rigid solid, such as a plaster. By filling the negative mold with the second molding compound, a positive impression of the original object is formed. Because the original object, and hence the negative impression molded therefrom, may be quite small, it is desirable to provide a means by which the second, positive molded impression can be extended so as to form a molded object that is better suited for display. It is further desired that the extension of the molded object be formed to produce a particular desirable appearance. In addition, in order to minimize handling of the negative mold, it is desirable to provide a system by which the positive impression can be formed without removing the negative mold from the container in which it is formed.